Can you test the video’s app on your iPad when it is connected through the GL.iNet router’s Mullvad VPN, not the iOS Mullvad VPN?
Also, if you open the iPad browser and go to https://whatismyipaddress.com/ and https://www.whatismyip.com/ when connected through the GL.iNet router’s Mullvad VPN, do you get the Mullvad server’s IP and location, not your ISP’s IP and location?
I usually do not have to explicitly specify the DNS and it automatically uses the VPN provider’s DNS. Note that the 172.70.xxx.yyy IPs are Cloudflare.